In the existing grease separation systems for catering wastewater, in the catering industry of Hong Kong, particularly in the small- or medium-scaled catering industry, horizontal-flow grease traps are generally used. However, there are only two grease trap chambers in such traps, and neither grease scrapers for removing grease in water surface nor means for separating food residue are provided. Accordingly, the grease separation efficiency is unsatisfactory, it is inconvenient and insufficient to remove grease in water surface, and it is likely to deposit food residue on the bottom of the trap. Due to no residue discharge pipe on the bottom, it is difficult and incomplete to remove residue in the trap. Consequently, the grease trap will often be blocked with foul smell. The cleanliness of the catering industry is influenced.